At present, current differential protection is widely used by power system as the main protection due to its good performance. The identifying criterion of a classical current differential protection is shown below:
                    {                                                                                                                                                                  I                        .                                            M                                        +                                                                  I                        .                                            N                                                                                        >                                  k                  ·                                                                                                                                  I                          .                                                M                                            -                                                                        I                          .                                                N                                                                                                                                                                                                                                                                                              I                        .                                            M                                        +                                                                  I                        .                                            N                                                                                        >                                  I                                      d                    ⁢                                                                                  ⁢                    m                    ⁢                                                                                  ⁢                    i                    ⁢                                                                                  ⁢                    n                                                                                                          (        1        )            
Such criterion is based on full component currents, which are the original measured currents including load current and fault component current based on superposition principle; however the solution also shows the following problems:                1) Low sensitivity and slow operation speed for heavy load conditions or high resistance faults;        2) Possible mal-trip for external fault with CT saturation;        3) Difficult to balance sensitivity, speed and reliability. Low threshold for good sensitivity and speed, the reliability will be worse. High threshold for good reliability, the sensitivity and speed will be worse.        
To resolve these problems, fault component based differential protection was proposed by researchers and manufactures. The algorithm of a typical fault component based differential protection is shown as below:
                    {                                                                                                                                          Δ                      ⁢                                                                                          ⁢                                                                        I                          .                                                M                                                              +                                          Δ                      ⁢                                                                                          ⁢                                                                        I                          .                                                N                                                                                                              >                                  k                  ×                                                                                                        Δ                        ⁢                                                                                                  ⁢                                                                              I                            .                                                    M                                                                    -                                              Δ                        ⁢                                                                                                  ⁢                                                                              I                            .                                                    N                                                                                                                                                                                                                                                                                              Δ                      ⁢                                                                                          ⁢                                                                        I                          .                                                M                                                              +                                          Δ                      ⁢                                                                                          ⁢                                                                        I                          .                                                N                                                                                                              >                                  I                                      d                    ⁢                                                                                  ⁢                    min                                                                                                          (        2        )            
It can be seen that such solution will have better sensitivity and speed than the classical current differential relay under the conditions of heavy load and high resistance fault generally, because the load currents are removed from the calculation. But existing fault component differential protection only improves the sensitivity and speed in some sense, it doesn't solve the reliability problem of CT saturation; moreover no effective solution which achieves good balance between sensitivity and reliability.
Similar to classical full component based differential protection; the classical fault component based current differential protection may mal-trip due to the external fault with CT saturation. In addition, both the two differential protections are difficult to achieve good balance between sensitivity, speed and reliability. If the sensitivity and speed are improved for identifying internal fault, it will reduce the reliability of identifying external fault. On the other hand, if the reliability of identifying external fault is improved, it will reduce the sensitivity and speed for identifying internal fault.
Consequently, existing solutions mentioned above are difficult to ensure both sensitivity and reliability of the differential protection as well as speed simultaneously. Due to the above mentioned problems, the present invention is to propose a method for identifying the fault by current differential protection and a device thereof.